Effects of Seedling Grade on Biomass Accumulation and Allometric Growth Across Developmental Stages in Pinus yunnanensis Franch. Seedlings

Pinus yunnanensis Franch. holds significant ecological value as a native conifer endemic to the southwestern uplands of China. The annual P. yunnanensis seedlings were categorized into three quality classes: Grade I, Grade II, and Grade III in accordance with the national specifications for seedling quality. We measured root, stem, needle, aboveground, and individual biomass at six stages from 240 to 690 days. We used the standardized major axis method to analyze allometric relationships among organs and with total biomass. Our results showed that Grade I seedlings had more stable allometric coordination over time. The lower grades showed more fluctuations and weaker coordination. Needles dominated biomass throughout the development period, roots grew faster in later stages, and stems grew slower but increased their share. This showed seedlings invest first in needles for carbon gain and then gradually build support and uptake structures. The biomass allocation and growth modes matured between 510 and 690 days. Biomass differed among seedling grades. Grade I seedlings show better growth stability and resource-use efficiency. They are suitable for nursery cultivation and afforestation. This study lays the groundwork for upgrading quality standards and perfecting silvicultural techniques. It also helps design precise tending measures to raise survival rates and early productivity. This work has practical value for plantation establishment, ecological restoration, and timber development in Southwest China.